Examples of expanded material made of plastics, particularly expanded materials prepared by an extruder, which have recently been marketed include polystyrene foam, polyethylene foam, and polypropylene foam. These expanded materials have found wide application in many fields because of their light weight, excellent insulation characteristics, excellent acoustic insulation characteristics and excellent cushioning properties as well as excellent moldability. On the other hand, these expanded materials are bulky and can be little decomposed in natural environment. Thus, these expanded materials accumulate in the natural world to cause the shortage of places to be reclaimed, pollute soil, rivers and ocean, raising a great social problem. As a countermeasure of inhibiting the environmental pollution there has been keenly desired the development of biodegradable polymers which can decompose with microorganisms in the soil and water and then get into a material chain system in natural environment to inhibit the environmental pollution.
Studies have been made of research and development of biodegradable polymers. For example, expanded materials of lactic acid based polymer such as polylactic acid and copolymer of lactic acid with hydroxycarboxylic acid other than lactic acid, polysaccharide such as corn starch and natural substance mainly composed of derivative thereof are the target of studies.
For the details of expanded materials of polysaccharide based natural substance, reference can be made to JP-A-5-39377 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the expanded materials disclosed in the above cited patent are disadvantageous in that they have a reduced surface smoothness and a deteriorated water resistance, restricting their use.
Lactic acid based polymers can be used with safety. As living body-absorbing polymers, they have been used as suture, bone internal fixation material, DDS (drug delivery system), etc. Further, since the lactic acid based polymers give a low combustion calorie when combusted, their development and use as polymers harmless to environment have been extensively studied in recent years.
Lactic acid based polymer expanded materials are disclosed in JP-A-4-304244, JP-A-5-140361, JP-A-6-287347, and JP-B-5-508669 (The term "JP-B" as used herein means an "examined Japanese patent publication"). These patents relate to expanded materials obtained by the extrusion of a lactic acid based polyester such as polylactic acid and copolymer of lactic acid with .epsilon.-caprolactone through an extruder.
These polylactic acid expanded materials have an excellent biodegradability and a high hardness but are disadvantageous in that they are brittle and weak against impact. Further, the copolymer of lactic acid with .epsilon.-caprolactone is relatively flexible but is disadvantageous in that it has a low heat resistance. Moreover, these lactic acid based polyesters generally have a great content of residual volatile matter, particularly residual lactide. Therefore, these lactic acid based polyesters are subject to thermal deterioration upon forming. Further, these lactic acid based polyesters tend to have a lower molecular weight and a lower viscosity, making it difficult to obtain an expanded material having a high expansion ratio. The resulting expanded materials have a reduced mechanical strength and a deteriorated heat resistance and storage stability. This is mainly because the residual lactide reacts with water in the air to produce an organic acid that causes the hydrolysis and breakage of polymer chains during forming.
A laminate of a lactic acid based polyester expanded material has been little reported. It is described as a formed product of lactic acid based polymer expanded material only in JP-A-6-287347, etc. However, if a L-polylactic acid is used as a surface non-expanded material described in the examples of the above cited patent, the resulting product has a reduced impact resistance. Further, if a blend of a poly-DL-lactic acid with a poly-L-lactic acid is used, the resulting product has a reduced heat resistance. If a copolymer of a poly-L-lactic acid with caprolactone is used, the resulting product has a deteriorated heat resistance and transparency.
Further, since the polymerization catalyst used in the preparation of these prior art lactic acid based polyester expanded materials is not deactivated after use, it then acts as a catalyst for decomposition reaction, lowering the molecular weight of the product during the preparation and forming of the laminate. Moreover, when the residual monomers are devolatilized away under reduced pressure while the polymer is melted, the molecular weight of the product is lowered, and monomers are produced from the polymer, eventually making the removal of monomers insufficient. As a result, the molecular weight of the laminate thus obtained is accelerately lowered by the action of an acid produced during storage, deteriorating the physical properties of the laminate in a short period of time.